Preheat the oven to 350 degrees. Spray a 9×13 cake pan with nonstick cooking spray.
Cream the butter and sugar with a handheld mixer until fluffy, about 2 minutes. Add the eggs, one at a time, incorporating into the batter after each one. Add the milk and vanilla and stir into the batter.
In a medium bowl, combine the flour, baking powder, salt, and cinnamon. Add the diced apples and stir to combine. Add dry ingredients with apples into wet ingredients and mix well.
Spread the thick batter into the pan. Bake for 45 minutes or until a toothpick comes out clean. Cut into 20 slices.